The ability to discriminate between two genes that differ by only a single base pair has led to important applications, particularly in the diagnosis of certain human genetic diseases as well as to the screening of mutations created by site-directed mutagenesis. The most general approach to this discrimination has been through the use of specific synthetic oligo hybridization probes. Oligo:DNA duplexes containing single base pair mismatches are significantly less stable than those which are perfectly base-paired. Under appropriate conditions, oligo probes will only hybridize to their cognate sequence and not to a sequence containing one or more non-complementary nucleotide. Thus, oligo hybridization can be used to determine the sequence of a short region of a DNA molecule.
Until recently, it has not been possible to discriminate between RNA molecules which differ by only a single nucleotide. Although oligo probes have been used to discriminate between highly related RNAs, never have single nucleotide differences been probed.
It has been shown that G:T mismatches in oligo:DNA duplexes are less destabilizing than other mismatches. Similarly, G:U mismatches in RNA:RNA duplexes are more stable than other mismatches. To optimally discriminate two genes which differ by a single transition mutation, oligo probes can be synthesized such that they form an A:C mismatch with the DNA of the non-complementary allele. Due to the fact that RNA is single stranded, it is not always possible to avoid an oligo probe forming a G:T or G:U mismatch with a non-complementary RNA when hybridizing oligo probes to two RNA molecules which differ by a single nucleotide. To discriminate the two RNAs on the basis of oligo hybridization, one must attempt to optimize the destabilizing effect of the mismatch formed.